Closed cell trench power MOSFET structure

ABSTRACT

A closed cell trench MOSFET structure having a drain region of a first conductivity type, a body of a second conductivity type, a trenched gate, and a plurality of source regions of the first conductivity type is provided. The body is located on the drain region. The trenched gate is located in the body and has at least two stripe portions and a cross portion. A bottom of the stripe portions is located in the drain region and a bottom of the cross portion is in the body. The source regions are located in the body and at least adjacent to the stripe region of the trenched gate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a trench power metal oxide semiconductor fieldeffect transistor (MOSFET) structure and a method to fabricate the same,and more particularly relates to a closed cell trench power MOSFETstructure and a method to fabricate the same.

2. Description of the Prior Art

Attending with the developing of high frequency circuit applications,the demand of high switching speed transistors is increasing. Thetransistor structure with higher switching speed is capable to reduceswitching loss so as to enhance power efficiency.

FIG. 1A is a schematic view of a typical striped cell trench powerMOSFET structure. As shown, the trench power MOSFET structure has anN-type heavily doped substrate 10, a N-type epitaxial layer 12, a P-typebody 13, a gate oxide layer 15, a trenched gate 16, and a plurality ofsource regions 17. The epitaxial layer 12 is formed on the substrate 10.The body 13 is formed on the epitaxial layer 12. The gate oxide layer 15encircles the trenched gate 16 to separate the trenched gate 16 and body13. The trenched gate 16 is composed of a plurality of stripe-shapedpolysilicon structures, which are located in the body 13 with apredetermined interval. A bottom of the stripe-shaped polysiliconstructure is located in the epitaxial layer 12 below the body 13. Thesource regions 17 located by the both sides of the stripe-shapedpolysilicon structures.

FIG. 1B is a schematic view of a typical closed cell trench power MOSFETstructure. As shown, the trench power MOSFET structure has an N-typeheavily doped substrate 20, a N-type epitaxial layer 22, a P-type body23, a gate oxide layer 25, a trenched gate 26, and a plurality of sourceregions 27. The epitaxial layer 22 is located on the substrate 20. Thebody 23 is located on the epitaxial layer 22. The gate oxide layer 25encircles the trenched gate 26 to separate the trenched gate 26 and theP body 23. The trenched gate 26 shows a network structure in the body 23to define a plurality of square areas. The bottom of the trenched gate26 is located in the epitaxial layer 22 below the body 23. The sourceregions 27 are located in the square areas defined by the trenched gate26.

The channel of the closed cell trench power MOSFET structure has a widthproportional to the boundary length of the square source regions 27. Thechannel width of the striped cell trench power MOSFET structure isproportional to a side length of the stripe-shaped source regions 17. Incontrast with the striped cell one, the closed cell trench power MOSFETstructure featuring a greater channel width per unit surface area has alower on-resistance (Ron).

However, as gate-to-drain capacitance (Cgd) is concerned, becausegate-to-drain capacitance is proportional to the bottom area of thetrenched gate 16,26, the trenched gate 26 of the closed cell trenchpower MOSFET structure occupies a greater surface area than the stripedcell one may result in a higher gate-to-drain capacitance.

In conclusion, the closed cell trench power MOSFET structure featureslower on-resistance but higher gate-to-drain capacitance in contrastwith the striped cell one. The feature of high gate-to-drain capacitancemay restrict the switching speed of the transistor structure and hinderthe development of high frequency electronic circuit applications.Accordingly, it has become an important issue in the field of thepresent invention to improve gate-to-drain capacitance of the closedcell trench power MOSFET structure.

SUMMARY OF THE INVENTION

It is a main object of the present invention to reduce gate-to-draincapacitance of the closed cell trench power MOSFET structure by reducingjunction area between the trenched gate and the drain region so as toenhance switching speed.

To achieve the above mentioned object, a closed cell trenchmetal-oxide-semiconductor field effect transistor (MOSFET) structure isprovided in the present invention. The closed cell trench MOSFETstructure comprises a drain region of a first conductivity type, a bodyof a second conductivity type, a trenched gate, and a plurality ofsource regions of the first conductivity type. The body is located onthe drain region. The trenched gate is located in the body and has atleast two stripe portions and a cross portion, wherein a bottom of thestripe portion is located in the drain region and that of the crossportion is located in the body. The source regions are located in thebody and at least adjacent to the stripe portion of the trenched gate.

A method to fabricate a closed cell trench MOSFET structure is alsoprovided in the present invention. The method comprises the steps of:(a) providing a drain region of a first conductivity type; (b) forming afirst doped region of a second conductivity type on the drain region;(c) forming a trench in the first doped region, the trench having atleast two stripe regions and a cross region, and a bottom thereof beinglocated in the drain region; (d) forming a gate dielectric layer lininginner surfaces of the trench; (e) forming a first polysilicon layer inthe trench, and the first polysilicon layer substantially filling thestripe region but having a concave at a middle of the cross region; (f)etching the first polysilicon layer to from a window to expose a bottomof the cross region; (g) forming a second doped region with the secondconductivity type adjacent to the bottom of the cross region through theetched first polysilicon layer; and (h) forming a second polysiliconlayer to fill the window.

In contrast with the traditional closed cell trench MOSFET structurewith a bottom of the trenched gate fully located in the drain region, inthe closed cell trench MOSFET structure of the present invention, onlythe bottom of the stripe portion of the trenched gate is located in thedrain region, and the bottom of the cross portion of the trenched gateis located in the body. Thus, the closed cell trench MOSFET structure iscapable to effectively reduce junction area between the drain and thegate but maintain the channel width of the MOSFET structure. As aresult, gate-to-drain capacitance of the closed cell trench MOSFETstructure can be reduced with on-resistance being kept low.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be specified with reference to itspreferred embodiment illustrated in the drawings, in which:

FIG. 1A is a cross-section view of a typical striped cell trench powerMOSFET structure;

FIG. 1B is a cross-section view of a typical closed cell trench powerMOSFET structure;

FIGS. 2A to 2J are schematic views showing a method to fabricate aclosed cell trench MOSFET structure in accordance with a firstembodiment of the present invention, wherein FIGS. 2I and 2J are topview and cross-section view of the closed cell trench MOSFET structure;

FIGS. 3A to 3B are top view and cross-section view of a closed celltrench MOSFET structure in accordance with a second embodiment of thepresent invention;

FIG. 4 is a schematic view of a closed cell trench MOSFET structure inaccordance with a third embodiment of the present invention; and

FIG. 5 is a schematic view of a closed cell trench MOSFET structure inaccordance with a fourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The idea of the present invention is to shield the bottom of the crossportion of the trenched gate with the body of the MOSFET structure so asto achieve the object of reducing the junction area between the trenchedgate and the drain region but having the width of the channel remained.In one embodiment of the fabrication method provided in the presentinvention, a polysilicon mask is fabricated to cover the bottom of thestripe region of the trench but leave the bottom of the cross region ofthe trench exposed by using the feature that the width of the crossregion is greater than that of the stripe region. Then, a doped regionhaving a conductivity type identical to the body is formed through thepolysilicon mask to shield the bottom of the cross portion of thetrenched gate.

FIGS. 2I and 2J are cross-section view and top view of a closed celltrench MOSFET structure in accordance with a preferred embodiment of thepresent invention, wherein FIG. 2I is a cross-section view correspondingto B-B′ cross section of FIG. 2J.

As shown in FIG. 2I, the closed cell trench MOSFET structure has a drainregion of a first conductivity type, a body 130 of a second conductivitytype, a trenched gate 172, and a plurality of source regions 180 of thefirst conductivity type. The drain region is composed of a heavily dopedsubstrate 100 and an epitaxial layer 120. The first conductivity typeand the second conductivity type may be N-type and P-type respectively.However, the present invention is not so restricted. The firstconductivity type and the second conductivity type may be P-type andN-type also. The body 130 is located on the epitaxial layer 120. Thetrenched gate 172 is located in the body 130. Also referring to FIG. 2J,the trenched gate 172 has at least two stripe portions 172 a and a crossportion 172 b. The bottom of the stripe portion 172 a is located in thedrain region, and the bottom of the cross portion 172 b is located inthe body 130. The source regions 180 are located in the body 130 andadjacent to the stripe portions 172 a of the trenched gate 172.

As shown in FIG. 2J, the stripe portion 172 a and the cross portion 172b of the trenched gate 172 are of substantially identical depth. But thebody 130 has a flat area 130 a and a protruding area 130 b on a lowersurface thereof. The flat area 130 a is corresponding to the stripeportion 172 a of the trenched gate 172 and has a depth smaller than thatof the stripe portion 172 a. The protruding area 130 b is correspondingto the cross portion 172 b of the trenched gate 172 and has a depthgreater than that of the cross portion 172 b. That is, the bottom of thestripe portion 172 a of the trenched gate 172 may be located in thedrain region below the body 130, but the bottom of the cross portion 172b of the trenched gate 172 is located in the body 130 respective to theprotruding area 130 b.

In the present embodiment, the stripe portion 172 a and the crossportion 172 b of the trenched gate 172 are of substantially identicaldepth. However, the present invention is not so restricted. According toanother embodiment of the present invention, the stripe portion 172 aand the cross portion 172 b of the gate 172 may be of different depth asthe body 130 corresponded to the protruding area 130 b is deep enough tocover the bottom of the cross portion 172 b of the trenched gate 172.

Moreover, in the present embodiment, the body 130 has a protruding area130 b on a lower surface thereof to have the bottom of the cross portion172 b of the trenched gate 172 located within the body 130. However, thepresent invention is not so restricted. FIGS. 3A and 3B show the secondembodiment of the closed cell trench MOSFET structure in accordance withthe present invention, wherein FIG. 3B is a cross-section viewcorresponding to the cross-section C-C′ in FIG. 3A. In this embodiment,the lower surface of the body 230 is a flat plane, but the stripeportion 272 a and the cross portion 272 b of the trenched gate 272 areof different depth. The bottom of the stripe portion 272 a is locatedbelow the lower surface of the body 230, but the bottom of the crossportion 272 b is located above the lower surface of the body 230.

In the present embodiment, the body 130 has a downward protruding area130 b on a lower surface thereof to shield the bottom of the crossportion 172 b of the trenched gate 172. But the present invention is notso restricted. As shown in FIG. 4, in accordance with a third embodimentof the present invention, the body 330 can be separated into a firstdoped region 330 a and at least a second doped region 330 b. The seconddoped region 330 b is located below the first doped region 330 a andisolated from the first doped region 330 a. The trenched gate 172 islocated in the first doped region 330 a. The bottom of the cross region172 b of the trenched gate 172 is located in the second doped region 330b.

FIGS. 2A to 2J are schematic views showing a method to fabricate aclosed cell trench MOSFET structure in accordance with a preferredembodiment of the present invention. FIG. 2B is a cross-section viewrespective to A-A′ cross section in FIG. 2A. Firstly, as shown in FIG.2B, a substrate 100 of a first conductivity type is provided and anepitaxial layer 120 of the first conductivity type is formed on thesubstrate 100 to compose a drain region. Thereafter, a body 130 of asecond conductivity type is formed on the drain region. Then, a trenched140 is formed in the body 130. The depth of the trench 140 is greaterthan that of the body 130 so that the bottom of the trench 140 islocated in the drain region.

Also referring to FIG. 2A, the trench 140 has at least two striperegions 142 and a cross region 144 located at the intersection of thetwo stripe regions 142. The width w1 of the stripe region 142 is smallerthan the width w2 of the cross region 144. Thereafter, as shown in FIG.2C, a gate dielectric layer 150 is formed to line the inner surface ofthe trench 140. Then, as shown in FIG. 2D, a first polysilicon layer 160is deposited in the trench 140. Since the width w1 of the stripe region142 is smaller than the width w2 of the cross region 144, it is capableto fully fill the stripe regions 142 with the first polysilicon layer160 of adequate thickness but leave a significant concave at the middleof the cross region 144.

Thereafter, as shown in FIG. 2E, the first polysilicon layer 160 isetched by using anisotropic etching technology to form a window 164corresponding to the concave in the first polysilicon layer 160 asmentioned above so as to expose the bottom of the cross region 144 butleave the bottom of the stripe region 142 being totally covered by theetched polysilicon layer 162.

Then, as shown in FIG. 2F, an ion implantation process is carried out toform a doped region 165 b of the second conductivity type adjacent tothe cross region 144 by implanting dopants of the second conductivitytype through the etched polysilicon layer 162. As a preferredembodiment, the ion implantation process can be carried out by using theetched polysilicon layer 162 as a mask, and the additional mask fordefining the position of the doped region 165 b can be skipped. Althougha doped region 165 a is also formed adjacent to the upper surface of thebody 130 in the present ion implantation process, the conductivity typeof the body 130 would not be altered for the conductivity type ofimplanted dopants is identical to that of the body 130.

Thereafter, as shown in FIG. 2G, a second polysilicon layer 170 isdeposited as a whole to fill the window 164 in the trench 140. Then, thesecond polysilicon layer 170 is etched back to leave the polysiliconstructure 172 within the trench 140 as the gate electrode of the MOSFETstructure. Finally, as shown in FIGS. 2I and 2J, a plurality of sourceregions 180 of the first conductivity type is fowled in the body 130 andat least adjacent to the stripe portion 172 a of the trenched gate 172.

It is worth noted that as shown in FIGS. 2A and 2B, the trench 140 inthe present embodiment has longitude portions and traverse portions todefine a plurality of square areas in the body 130. The source regions180 are located in the square areas. The portion of the trench 140 atthe intersection of the longitude portions and the traverse portions isdefined as the cross region 144, and the rest of the trench 140 isdefined as the striped regions 142. As shown in the top view of thesemiconductor structure, the cross region 144 shows a square shape witha side length substantially identical to the width of the stripe region142. However, the present invention is not so restricted. The crossregion 144 may have an upper surface of different shapes according tothe layout of the trench 140. Referring to FIG. 5, in the fourthembodiment of the present invention, the trench 440 is sorted of threegroups of different extending directions to define a plurality oftriangles areas in the body 430. The source regions (not shown) areformed in the triangles areas. The cross region 444 of the trench 440 inthe present embodiment has an upper surface of hexagonal shape with awidth w4 greater than a width w3 of the stripe region 442 and has a sidelength substantially identical to the width w3.

In addition, as shown in FIGS. 2F and 2G, a drive-in step is carried outafter the step of forming the second doped regions 165 b below the crossregion 144 of the trench 140 in accordance with the present embodiment.The drive-in step may extend the second doped region 165 b to connectwith the body 130. Thereby, the body 130 with the downward protrudingarea on the lower surface thereof is provided. However, the presentinvention is not so restricted. The above mentioned drive-in step is anoptional step. Referring to FIG. 4, in accordance with the thirdembodiment of the present invention without the drive-in step beingcarried out, the body 330 has a first doped region 330 a and a seconddoped region 330 b, and the second doped region 330 b shielding thebottom of the cross region 144 of the trench 140 is isolated from thefirst doped region 330 a.

In contrast with the traditional closed cell trench MOSFET structure ofFIG. 1B, which has a bottom of the trenched gate 26 being totallylocated in the drain region, referring to the closed cell trench MOSFETstructure in accordance with the present invention as shown in FIG. 2I,only the bottom of the stripe portion 172 a of the trenched gate 172 islocated in the drain region, and the bottom of the cross portion 172 bis left in the body 130 to reduce the junction area between the drainregion and the gate electrode but maintain the width of the channel. Asa result, by using the closed cell trench MOSFET structure in accordancewith the present invention, gate-to-drain capacitance can be reduced andon-resistance can be kept low.

While the preferred embodiments of the present invention have been setforth for the purpose of disclosure, modifications of the disclosedembodiments of the present invention as well as other embodimentsthereof may occur to those skilled in the art. Accordingly, the appendedclaims are intended to cover all embodiments which do not depart fromthe spirit and scope of the present invention.

1. A closed cell trench metal-oxide-semiconductor field effecttransistor (MOSFET) structure, comprising: a drain region of a firstconductivity type; a body of a second conductivity type, located on thedrain region; a trenched gate, located in the body and having at leasttwo stripe portions and a cross portion, wherein a bottom of the stripeportions is located in the drain region and a bottom of the crossportion is located in the body, and wherein the body has a flat area andat least a downward protruding area on a lower surface thereof, and thebottom of the cross portion is arranged in the correspondingly downwardprotruding area; and a plurality of source regions of the firstconductivity type, located in the body and at least adjacent to thestripe portions of the trenched gate.
 2. The closed cell trench MOSFETstructure of claim 1, wherein the trenched gate has a depth greater thanthat of the body corresponding to the flat area.
 3. The closed celltrench MOSFET structure of claim 1, wherein at least one of the stripeportions and the cross portion of the trenched gate have substantiallyidentical depth.
 4. The closed cell trench MOSFET structure of claim 1,wherein the body has a first doped region and a second doped region islocated below the first doped region and isolated from the first dopedregion, the trenched gate is mainly located in the first doped regionbut with the bottom of the cross portion being located in the seconddoped region.
 5. The closed cell trench MOSFET structure of claim 1,wherein the cross portion has a polygonal upper surface with a number ofsides greater or equal to four.